Optical pulses have an electric field associated therewith. As shown in FIG. 1, the electrical field can be described as a high-frequency oscillation, known as the “carrier” (or “carrier-wave”) 12. The carrier 12 is contained within a lower-frequency “envelope” 14. As shown, the carrier peak magnitude 16 and the envelope peak magnitude 18 are not always aligned, and the difference in relative position between the carrier and the envelope is known as the offset phase. The offset phase can shift as the optical pulses pass through a medium in which the carrier and envelope propagate at different speeds.
Only recently, it became possible to completely control the temporal evolution of the electric light field of a train of mode-locked laser pulses. Mastering the manipulation of phase and magnitude of the electric field has been made possible by technological advances in femtosecond laser technology and nonlinear optics together with ground-breaking ideas and insights in the field of precision spectroscopy with pulsed laser sources. This unprecedented high level of control enables a wide range of new applications in science and technology. Time domain applications focus on studies of physical phenomena directly depending on the electric field rather than on the pulse envelope only. Examples of such applications include carrier-wave Rabi-flopping, quantum interference of photocurrents, photoemission from metal surfaces, or electron emission from ionized atoms. Furthermore, attosecond physics has been made accessible by using carrier-envelope-offset-frequency-controlled femtosecond pulses to generate coherent light in the deep UV and X-ray spectral regions in a well-controlled manner. Analogously, the high degree of control of the electric field is also very beneficial for applications in the frequency domain where the laser spectrum, composed of discrete longitudinal modes, is being used for pioneering experiments in optical frequency metrology.